Methods and Systems for Leak Testing

ABSTRACT

Leak rate testing is an engineering challenge where on the one hand, engineers must meet strict leak rate standards on a wide range of products and systems from semiconductor packages through medical product packaging to chemical storage vessels and liquid/gas handling systems. On the other hand, they have to make the leak testing process low cost and independent of operator whilst in many applications making the process automated and fast as this step may otherwise become a manufacturing bottleneck. Accordingly embodiments of the invention address manufacturing requirements by providing for high accuracy flow based leak testing of large volumes, providing adaptive techniques for use during testing, providing equivalent circuit modeling techniques allowing optimization and parameter extraction to be simulated prior to manufacturing commitment, and providing for the automatic tuning of setup parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication U.S. 61/655,624 filed Jun. 5, 2012 entitled “Methods andSystems for Leak Testing”, the entire contents of which are included byreference.

FIELD OF THE INVENTION

The present invention relates to leak testing and more specifically toenhancing accuracy, speed and defect detection.

BACKGROUND OF THE INVENTION

Leak rate testing is an engineering challenge where on the one hand,engineers must meet strict leak rate standards on a wide range ofproducts and systems from semiconductor packages through medical productpackaging to chemical storage vessels and liquid/gas handling systems.On the other hand, they have to make the leak testing process low costand independent of operator whilst in many applications making theprocess automated and fast as this step may otherwise become amanufacturing bottleneck. To address these conflicting demands engineersmust understand all aspects of the leak testing process. It is alsoimportant that it is understood what a product leak is. This common termis not always well defined but is basically a material flow from or intoa product (a control volume) within a predetermined period of time whichis in excess of an allowable limit. Product leaks are typically causedby open flow paths, such as pinholes, broken seals or material porosityand in most cases the product leak is a very small flow. It is thisprocess of quantifying a product leak and eliminating products basedupon the measured leak that is known as leak testing. In thepharmaceutical, medical, and food industries, it is called package orseal integrity testing.

Leak testing requires the accurate measurement of very small flow ratesof a gas or liquid within what may in some instances be made for a largevolume or in others rapidly. Typically measured as a flow rate, such asstandard cubic centimeters per minute (sccm) or cubic centimeters ofhelium per second (cc/s He) and millibar liters per second (mbar·l/s)and may, according to application, range from 10⁻³ to 10⁻¹² mbar·l/s. Insome cases, the leak flow rate is correlated to a “virtual pinhole,” toquantify the size of potential defects. For example, to preventcontamination, a sterilized medical package must be sealed such that a“virtual pinhole” in the product is smaller than the size of thesmallest microorganism (commonly 0.2 μm in diameter). This theoreticalpinhole dimension and the leak flow rate are correlated to each other.

Irrespective of the actual component, device, product and/or systembeing leak tested the balance of speed, accuracy, and cost exists.Whilst increased speed (reduced time) of leak testing reduces cost perunit a corresponding reduction in accuracy from this may lead toincreased costs from yield (as rejections may actually have passed withan increased accuracy test) and/or product failures and customer impact(as products failing at the customer which were incorrectly passedimpact yield, customer satisfaction, and in critical cases may leaddirectly to damages payable by the manufacturer. Accordingly there isconsiderable benefit to manufacturers in increasing accuracy, increasingspeed, and increasing defect detection in manufacturing leak testing.

Additionally some scenarios present further issues, such as for examplelarge volume leak testing. Leak testing with large part volumes in the30 L (˜7.9 gallons) and above range, creates additional challengesincluding temperature sensitivity and pressure sensitivity. Take theexample of a 77 L part, approximately 19 gallons, then a conventionalflow based leak test would uses a flow meter in series with a pressureregulator such as shown in FIG. 1. The Test Pressure Regulator R2 115keeps the pneumatic pressure at the desired test pressure and the FlowSensor F1 130 measures the flow to the Device under Test (DUT) 140. Thetest pressure regulator 115 is typically vented to atmospheric and thisforms the basis for the test pressure. For example if the atmosphericpressure is 14.7 psi and the regulator is set to 7.5 psi the absolutetest pressure is (14.7+7.5)=22.2 psi. Accordingly as the atmosphericpressure varies so does the absolute output pressure of the regulator.

Referring to Table 1 there is shown the resulting flow measurement ifthe part volume is 77 L and the atmospheric pressure varies by 0.002psi. The flow meter will have to allow 6.9 cc to flow in order to allowthe test chamber pressure to change. This is a large error if the goalis to read down to flow rates of 2 sccm reliably.

TABLE 1 Calculation of Volume Change due to Atmospheric Pressure ChangeDescription Value Unit Atmospheric Pressure Change 0.002 psi Volume ofPart 77 L Test Pressure 7.5 psi P absolute 14.7 psi Volume Change due toAtmospheric 6.9 Cc Pressure Change

As depicted in FIG. 1 the Test Pressure Regulator R2 115 and Flow SensorF1 130 form part of an overall pneumatic circuit with DUT 140. Air iscoupled from a source, commonly referred to as the shop air, filteredwith filter 105 and pressure regulated with Regulator R1 110 beforebeing coupled to the Test Pressure Regulator R2 115. The output of theTest Pressure Regulator R2 115 is coupled to the Flow Sensor F1 130 viaSupply Valve V3 120 and Flow Test Valve 125 and therein after the FlowSensor 130 to DUT 140 wherein an Absolute Pressure Sensor P1 135 is alsocoupled together with a Calibration Orifice 145 allowing calibration tobe performed via a non-return valve. The output of the Supply Valve V3120 is also coupled to Fill Valve V1 150 and Exhaust Valve V2 155 whichis also coupled to the output of the Flow Sensor 130 and Exhaust 160.

Accordingly through the appropriate sequencing of these valves the DUT140 may be filled, pressurized and tested before being exhausted to airfor de-coupling and another DUT 140 attached. Referring to FIG. 2 theresulting flow output (sccm) and ambient pressure (×2000 and DC removed)are depicted for such a DUT with volume 77 L over a period of 2000seconds.

Such issues combined with the continued drive for more accurate leaktest results, due to increased attention to quality, means that newcontrol/measurement techniques are required. Accordingly embodiments ofthe invention address manufacturing requirements by providing for highaccuracy flow based leak testing of large volumes, providing adaptivetechniques for use during testing, providing equivalent circuit modelingtechniques allowing optimization and parameter extraction to besimulated prior to manufacturing commitment, and providing for theautomatic tuning of setup parameters.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

SUMMARY OF THE INVENTION

It is an object of the present invention to enhance leak testing andmore specifically to enhancing accuracy, speed and defect detection.

In accordance with an embodiment of the invention there is provided asystem for testing a device under test (DUT) comprising:

-   a) filling the DUT with a fluid to a predetermined pressure using a    first regulator R2 and first valve V1 disposed in series between a    source of the fluid and the DUT;-   b) measuring with a high accuracy flow controller F1 a measured flow    waveform; and-   c) calculating with a microprocessor a leak rate for the DUT in    dependence upon at least the measured flow waveform.

In accordance with an embodiment of the invention there is provided amethod of testing a device under test (DUT) comprising:

-   a) fast filling the DUT with a fast fill regulator R2 via a fast    fill valve V1 at a pressure higher than the required final test    pressure;-   b) reducing the pressure of the DUT using fast fill regulator R2 to    required final test pressure;-   c) measuring DUT pressure using an absolute pressure sensor P2;-   d) setting a control set-point of an absolute test pressure    regulator R1 in dependence upon the measured DUT pressure;-   e) filling the DUT using absolute test pressure regulator R1 via a    fill valve V5 after closing fast fill valve V1; and-   f) switching to flow testing with high accuracy flow controller F1    via flow text valve V3 after closing fill valve V5;-   g) measuring with the high accuracy flow controller F1 a measured    flow waveform; and-   h) calculating with a microprocessor a leak rate for the DUT in    dependence upon at least the measured flow waveform.

In accordance with an embodiment of the invention there is provided amethod comprising:

-   an absolute pressure sensor in communication with a fluidic system    to be coupled to the DUT;-   a high accuracy flow controller and high resolution flow sensor    comprising a predetermined portion of the fluidic system and    disposed between a fluid source and the DUT;-   a high resolution low noise data acquisition circuit coupled to the    absolute pressure sensor for receiving the output of the absolute    pressure sensor and converting it to an electrical signal; and-   a controller receiving the output of the high resolution low noise    data acquisition circuit and controlling the flow of the high    accuracy flow controller with integral flow sensor in dependence    upon at least the processed output of the absolute pressure sensor.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 depicts a large volume leak rate test system according to theprior art;

FIG. 2 depicts the flow output (sccm) and ambient pressure (×2000 and DCremoved) for a 77 L test part over a period of 2000 seconds;

FIG. 3 depicts a large volume leak rate test system according to anembodiment of the invention;

FIG. 4 depicts a pneumatic system according to an embodiment of theinvention employing multistage pressure control;

FIG. 5 depicts an exemplary parametric model of a leak circuit forestablishing leak rate test parameters according to an embodiment of theinvention;

FIG. 6 depicts a plot of leak test error versus test phase start timeindicating the determination of an optimal start time for leak ratetests according to embodiments of the invention;

FIGS. 7A and 7B depict schematics of an overall leak rate test cycle forpressure decay and flow testing respectively;

FIG. 8 depicts a screen shot from an automated leak rate systemaccording to an embodiment of the invention;

FIG. 9 depicts applying signature analysis to a leak rate measurementaccording to an embodiment of the invention;

FIG. 10 depicts a corrected leak rate and original measured leak ratesas the result of temperature compensation according to an embodiment ofthe invention;

FIGS. 11A and 11B depict establishing a DUT's temperature from the rateof change of flow in a pressure decay curve according to an embodimentof the invention;

FIG. 12 depicts a pressure versus time waveform as exploited by a fullcurve analysis leak tester according to an embodiment of the invention;

FIG. 13 depicts a schematic of a system to applying signature analysisto the leak testing process according to an embodiment of the invention;

FIG. 14 depicts applying signature analysis the very beginning of thepressure curve to detect unique leak tester and DUT defects like sensornoise and offset according to an embodiment of the invention;

FIG. 15 depicts applying signature analysis to a portion of the pressurecurve to detect unique DUT defects like seal movement according to anembodiment of the invention;

FIG. 16 depicts fitting a model to the fill portion of the pressurecurve according to an embodiment of the invention;

FIG. 17 depicts early DUT rejection with signature analysis according toan embodiment of the invention; and

FIGS. 18A and 18B depict stabilization reduction using overpressure,showing pressure and flow curves respectively according to an embodimentof the invention.

DETAILED DESCRIPTION

The present invention is directed to leak testing and more specificallyto enhancing accuracy, speed and defect detection.

The ensuing description provides exemplary embodiment(s) only, and isnot intended to limit the scope, applicability or configuration of thedisclosure. Rather, the ensuing description of the exemplaryembodiment(s) will provide those skilled in the art with an enablingdescription for implementing an exemplary embodiment. It beingunderstood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope asset forth in the appended claims.

A “device under test” (DUT) as used herein and throughout thisdisclosure, refers to an item being tested for a leak. This includes,but is not limited to, devices, components, packages, packaging,containers, piping systems, individual elements, sub-systems and systemsrequiring that they isolate the interior from the ambient exterior. ADUT may or may not be intended to form part of a hydraulic, pneumatic,or fluidic system.

A “leak test” as used herein and throughout this disclosure, refers toprocess and/or method of determining whether a DUT has a leak which mayfor example comprise placing the DUT under a positive pressure relativeto its environment and determining attributes of the pressure/flow todetermine the rate of leakage from the DUT. However, leak test as usedherein is not limited to positive pressure testing and may include, butnot be limited to, negative pressure leak testing wherein DUTs cannot beplaced under positive pressure and helium leak testing. Such testing maybe performed with a fluid including for example air, nitrogen, heliumand other gases as well as water, silicone oil, and other liquids.

Traditionally leak testing was performed by using immersion testing orsniffing. In the former the DUT would be immersed into liquid and leaksidentified by observing bubbles. Such testing was good for gross leaksin mechanical systems but is limited where very low leak rates arerequired as such leak rates imply very long bubble formation.Additionally in many instances an inert material would have to beemployed in order not to damage the actual components under test.Sniffing in contrast comprised pressurizing a DUT with a material notpresent in the normal environment, typically helium gas, and using asensor to identify the presence of helium external to the DUT. However,this again was generally qualitative and for large systems required anoperator to physically move the sniffer along and around the DUT. Forsmall packages such as within electronics and opto-electronicsvariations of this evolved to measuring the helium within a chamber heldunder vacuum with the DUT inside.

Such helium leak rate testing falls into a category of testing commonlyknown as “amount of leak methods” which includes hard vacuum inside/out;hard vacuum outside/in; accumulation; carrier gas; mass flow; andResidual Gas Analysis which provide together measurable leak rates aslow as 10⁻⁹ std. cc/sec although some such as mass flow provide limiteddynamic ranges to approximately 10⁻² std. cc/sec when compared to otherssuch as hard vacuum. Another category of techniques are the “effect ofleak methods” which involve measuring the effect the leak has on somephysical quantity or quantities inside or around the DUT. In themajority of cases this quantity is pressure. Such techniques includepressure decay, pressure differential, pressure increase, and vacuumdelay and offer measurable leak rates down to approximately 10⁻³ std.cc/sec.

Within the following sections different approaches to enhancingaccuracy, speed and defect detection for leak rate testing areidentified according to embodiments of the invention. Whilst suchapproaches may be discussed in respect of one particular leak rate testmethodology it would be evident to one skilled in the art that suchapproaches may be applied to other methodologies without departing fromthe scope of the invention.

1. Software Correction:

Measurements of characteristics of sensors are performed and employedwithin an algorithm to compute a corrective waveform and/or correctivecoefficient used to determine the true flow and remove errors induced byvarious factors present within the measurement system. For example amethod according to an embodiment of the invention may comprise thesteps of:

-   -   1. Measure pressure with an absolute pressure sensor with high        resolution to generate an absolute pressure waveform;    -   2. Compute the derivative of the absolute pressure waveform;    -   3. Compute an effective flow due to the absolute pressure change        waveform using the data obtained from the absolute pressure        sensor in dependence upon the known DUT volume and absolute        pressure, see for example Equation (1);    -   4. Subtract this effective flow from the measured flow waveform;    -   5. Measure corrected, real, flow due to leak(s).

$\begin{matrix}{{Flow}_{CORR} = {\frac{Vol}{P_{A}} \times \frac{\delta \; P_{A}}{\delta \; t}}} & (1)\end{matrix}$

2. Absolute Pressure Control:

According to this embodiment of the invention a control method on theflow to keep the absolute pressure constant wherein a feedbackcontroller, such as a Proportional-Integration-Derivative (PID)controller, is employed to actively control the flow going into the DUTto keep the absolute pressure constant. The resulting flow value is thetrue leak rate of the DUT.

An example of a pneumatic electrical circuit providing such activecontrol according to an embodiment of the invention is depicted in FIG.3. The main elements of this circuit being:

-   -   1. P1 absolute test pressure sensor 335;    -   2. Feeding the output of the P1 absolute test pressure sensor        335 into a high resolution low noise data acquisition input        circuit with software based processing, depicted as High        Resolution Filtering and Processing 345;    -   3. A high accuracy flow controller and flow sensor 315 replaces        the flow meter, typically with limited flow; and    -   4. A PID controller 350 controls the high accuracy flow        controller with flow sensor 315 to control the flow based on the        processed output of the absolute pressure sensor 335.

Accordingly an exemplary manufacturing leak test process sequence usingthe pneumatic electrical circuit of FIG. 3 would proceed as follows:

-   -   1. Fill the DUT with the R2 regulator through V1 Fill 355 to get        the part to the desired test pressure;    -   2. Wait a predetermined period of time to allow the DUT to        stabilize;    -   3. Perform a high resolution measurement of the absolute test        pressure using P1 absolute test pressure sensor 335 and use this        as the set point for the subsequent steps below;    -   4. Close V1 Fill 355;    -   5. Engage the PID controller 350 to close the loop and keep the        test pressure at the value determined in step 3 within the        accuracy of electronic circuit comprising High Resolution        Filtering and Processing 345 and PID controller 350;    -   6. Measure the resulting control flow signal and determine the        DUT leak from this value with any required signal processing.

Embodiments of the High Resolution Filtering and Processing 345 by theinventors achieve a resolution of approximately 25×10⁻⁶ psi therebyallowing the PID controller 350 to close the feedback loop and maintainthe pressure constant to within this resolution.

Stabilization Step Removal:

One of the longest delays in leak testing is the stabilization phase. Inthis phase the heat that has been added to the air adiabatically throughthe compression of the air inside the DUT during the fill portion of theleak test is dissipating into the inside surface of the DUT. As this airinside the DUT cools back down to the inside surface temperature thepressure in the DUT drops in a pressure decay test or an increase flowis measured in a flow based test. This process can take time and iscontrolled by the physical characteristics of the inside of the DUTincluding volume of air, inside surface area, inside surface geometryand inside surface finish.

An issue with absolute pressure regulation is the signal to noise ratioas anything that decreases this will aid in decreasing the stability ofthe absolute pressure and thus the stability and accuracy of the leaktest results. Highest resolution can be obtained by using a differentialpressure transducer between a reference vessel and the DUT. In a methodaccording to an embodiment of the invention the reference volume wouldbe filled to the same pressure as the DUT test pressure then sealed off.The control algorithm, for example PID, would then act to close thepressure regulation loop based upon the difference between thisreference volume and the DUT. Accordingly, the differential pressuresensor can have a very low full scale range, for example 0.1 psi therebygiving the control electronics a large signal to noise ratio andresulting in increased stability of establishing an absolute pressure.

An exemplary sequence of events according to an embodiment of theinvention being:

-   -   1. Using a gauge fast fill regulator fill the DUT and the        reference volume to the same pressure.    -   2. Seal the reference volume so that it does not change in        pressure.    -   3. Employ a low full scale differential pressure sensor that        measures the pressure difference between the reference volume        and the DUT.    -   4. Switch control to a fine regulator that is under a control        loop with the differential pressure sensor as the feedback input        and a value of zero as the set point input.    -   5. Now the control algorithm has a large signal to noise ratio        input and can control the pressure with greater accuracy.

3. Multi-Stage Pressure Control to Optimize Fill Time:

Amongst the factors affecting overall cost of leak testing is equipmentutilization wherein reducing overall test time increases this metric fora manufacturing process step. Accordingly, a multi-stage active controlalgorithm according to embodiments of the invention allows anoptimization of the fill process which may be partitioned for exampleinto a fast fill stage, an adiabatic stabilization stage, and test time.Each phase benefits from unique control parameters which may varyaccording to the characteristics of the DUT.

According to embodiments of the invention such a multi-stage fill mayemploy, such as described above, multiple flow controllers with flowsensors which are controlled from PID controllers which each executeindividual PID algorithms coupled from digital processing circuits, suchas High Resolution Filtering and Processing 345, which receive theoutput of an absolute test pressure sensor. Alternatively multiple PIDcontrollers may receive their input from a single digital processingcircuit or a single PID controller and flow controller with flow sensormay be used wherein the PID algorithm changes for each stage based uponpredetermined conditions being met. Accordingly, each PID algorithmallows for actively setting the unique optimal gains for each phase ofthe leak test.

Stabilization Step Removal:

As discussed supra in respect of Section 2 one of the longest delays inleak testing is the stabilization phase. According to an embodiment ofthe invention the fill pressure is modified to increase the rate atwhich the heat is transferred out of the air inside the DUT to theinside surface of the DUT. The time taken by the stabilization phase isthe time it takes to remove all the added heat due to the adiabaticprocess. By temporarily increasing the pressure in the DUT for a periodof time the rate of heat transfer can be increased, thus reducing theoverall time to finish the stabilization phase. Referring to FIG. 18Athere are shown various times of this over-pressure stage from 20seconds to 30 seconds. FIG. 18B shows the resulting flow curves.Referring to curve 1215100084 then there is shown a time of 29.5 secondswhere the flow is stable at 195 seconds other if over pressure is notused the stabilization takes over 255 seconds.

Method 1: An exemplary sequence of events may therefore include:

-   -   1. Overfill the part to a pressure that is higher than the final        required test pressure.    -   2. Keep this higher pressure until all of the heat required at        the lower pressure has been transferred, this may be determined        through experimentation or automatically through automatic        algorithms such as described within this specification.    -   3. Reduce the pressure to the required test pressure.    -   4. Wait the shorter period of time for final stabilization.    -   5. Measure the leak now at the earlier point in time.

Method 2: Similar to Method 1 above but the fill pressure is modifieddynamically in an adaptive loop that monitors the current adiabatic rateof heat transfer and modifies the pressure dynamically in a control loopto achieve the fastest stabilization. The control algorithm would setthe pressure high then monitor the effective flow rate of change, forexample, and then modify the pressure to arrive at zero flow rate ofchange at the desired test pressure with stabilization complete wherethe flow is stable and not changing.

An exemplary sequence of events may therefore include:

-   -   1. Overfill the part to a pressure that is higher than the final        required test pressure.    -   2. In a control loop monitor the rate of change of the flow due        to stabilization heat transfer, adjust the pressure from over        fill pressure to test pressure using the stabilization flow as        the input.    -   3. Control the pressure to arrive at the test pressure with a        flow rate of change of zero.    -   4. Wait a shorter period of time for final stabilization.    -   5. Measure the leak now at the earlier time

4. Absolute Pressure Regulation:

Flow based leak testing techniques according to the prior art exploitgauge pressure regulators wherein the absolute output pressure of theregulator varies with the absolute atmospheric pressure. According toembodiments of the invention such gauge pressure regulators are replacedby absolute pressure regulators such that increase flow based leakdetection accuracy is obtained by removing the error induced in leaktesting due to atmospheric pressure changes.

5. Multi-Stage Pressure Control:

According to this embodiment of the invention increased accuracy andreduced detection time are provided through the application of amulti-stage pressure control methodology. Such an approach beingbeneficial to achieve high precision leak accuracy in large volume leaktesting.

Accordingly, multi-stage electronic pressure regulators, such as forexample those described above and in respect of FIG. 3 employing PIDcontrollers, are employed to fill the DUT rapidly and then keep theabsolute pressure constant. Such an approach requires unique high flowand fine flow controllers using PID control loops. Accordingly, a highflow regulator closes its loop based upon a gauge pressure sensor tofill the DUT rapidly to a set gauge test pressure. Next a fine flowcontroller closes a PID loop based upon an absolute pressure sensorwherein the set point of the fine flow regulator is set at the resultingabsolute pressure that the high flow regulator reaches. This allows theuse of a low flow capability, but precise flow regulator.

By appropriate design and implementation to address factors that impactthe multi-stage control loops such as ground loops and noise sources forexample, the inventors have established multi-stage pressure controlsystems that reduce the ripple on the absolute pressure to very lowlevels (<0.0001 psia for example). The fine flow regulator isimplemented with a design having an optimal valve size for low ripplewhilst the PID controller provides a gain circuit that integrates noisereduction circuits.

Now referring to FIG. 4 there is depicted an example of a multi-stagepressure control approach according to an embodiment of the invention.Employing this pneumatic electrical circuit to provide the required leaktesting may follow the following sequence of steps:

-   -   1. Start fast fill of DUT with R2 Fast Fill Regulator 435 at        higher gauge pressure to overcome hose resistance;    -   2. Reduce pressure using R2 Fast Fill Regulator 435 to final        gauge test pressure;    -   3. The current DUT pressure is measured accurately using the        absolute pressure sensor 485;    -   4. Set control set-point for the very fine control absolute        regulator R1 Test Pressure Regulator 450;    -   5. Start fine bypass fill of the DUT using R1 Test Pressure        Regulator 450 and V5 Fill 445;    -   6. Switch to flow testing through V3 Flow Test 460 now the fine        regulator R1 Test Pressure Regulator 450 controls the pressure        and ensures extremely low ripple in the absolute pressure.    -   7. The flow can now be measured using F1 flow meter 455

6. Automatic Generation of Parametric Model of Leak Circuit:

Traditionally the setup of the many adjustable parameters within a leaktest which can include fast fill time, fast fill pressure, fill time,stabilization time, test time and exhaust time for example have beendone by trial and error. Different values are tried and adjustedmanually based upon the experience of the leak test operator or limitedanalysis. Some of these durations are shown in the different zonesdepicted in FIGS. 7A and 7B. In many manufacturing environments theproduction line leak testing tends to be one of the longer tests thatare performed. This puts pressure on finding the optimum adjustment ofthe leak test parameters to achieve the desired results in respect ofaccuracy etc with the minimum overall test time.

The manual method of adjustment is prone to inaccuracies for manyreasons. The outcome is dependent on the experience of the personsetting up the leak test. It takes time to keep testing various setups,and then gather statistical data to determine the repeatability for eachsetup. There are so many parameters to adjust simultaneously and theyall have complex interrelations that it is very challenging to reach theoptimum configuration that yields the desired accuracy in the leastamount of time.

According to embodiments of the invention a leak test systemautomatically determines these parameters for optimum accuracy with theminimum test time. Various full waveform signature analysis techniquesmay be used together with different leak test operations and sequencesto determine a mathematical model of the tester, part under test and thecritical pneumatic pluming. This model is then used to determine theoptimum leak test parameter setup that will yield the desired accuracyin the fastest time possible. Accordingly, the pneumatic circuit can bemodeled as an electrical circuit such as depicted by equivalent circuit500 in FIG. 5 wherein flow restricting elements such as valves, hoses,fittings, flow meters, source resistance of regulators, are modeled asresistors, the DUT, hoses and other elements of the system with volumeare typically modeled as capacitances, atmospheric changes and inputregulator pressure are modeled as voltage sources, and thermal aspectscan be modeled as resistors and capacitors representing thermalresistance and capacitance. By applying multiple and various excitationsof the leak system overall the model parameters may be calculated fromthe resulting system responses.

Examples of excitations that may be applied and varied may include, butare not limited to, filling cycles, pressure changes, flow control, andtemperature changes. The model can include elements such as regulator,valves, hoses, DUT, leak, adiabatic process, and thermal time constantsfor example. Such excitations may be automatically generated and theresulting flow rates, pressures, etc. similarly automatically measuredwherein these measurements may then be employed as inputs in algorithmswhich may then be applied to the data to automatically establishparameters for different elements within the leak test system. Forexample the volume of a DUT may be established using the two processesidentified below such that the actual physical volume of the DUT ratherthan a design value may be employed thereby increasing the accuracy ofthe measurements.

Process A:

-   -   1. Obtain P_(A) with an absolute pressure sensor;    -   2. Calculate δ²P_(A), double derivative of P_(A);    -   3. Calculate δF, single derivative of the flow;    -   4. Determine the correlation between δ²P_(A) and δF;    -   5. Calculate least squares slope    -   6. Remove the absolute pressure part and thereby derive the        volume of the DUT.

Process B:

-   -   1. Change the P_(A) for the DUT;    -   2. Measure flow rate F;    -   3. Calculate net flow, ∫F, and use this along with the absolute        pressure to thereby derive the volume of the DUT.

7. Calculate Optimal Test Parameters from Parametric Model:

Once, an equivalent parametric model of the leak test system has beenestablished, such as described above in respect of Section 6 andassociated FIG. 5 then the derived parametric model may be simulatedusing one or more predetermined algorithms to determine the response ofthe parametric model and therein the response of the actual pneumaticcircuit to the simulated operation. Accordingly, the parametric modelcan be used to determine optimal system timings for different aspects ofthe leak test process including for example fast fill, fill,stabilization, test, and exhaust within the desired overall operatingparameters such as overall test time and gauge repeatability andreproducibility.

Accordingly an autotune process comprises the following steps:

-   -   A) Controlling the test system with various setups and analyzing        the resulting curves with signature analysis to determine the        mathematical model, using electrical equivalents, for example,        of the test system with the test part;    -   B) Determining from the mathematical model the optimum test        configuration for the accuracy and or time requirements required        of the system. This includes the fast fill pressure and time,        fill time, stabilization time, test time and exhaust time.    -   C) Chose the ideal system setup using the model to optimize the        accuracy and reduce time of the leak test    -   D) Determine the leak of the part.

In many configurations within the prior art determining the leak wasbased upon assumptions and best practice scenarios rather thanexperimental data. According to embodiments of the invention themeasurements and parameter setup of DUTs for leak rate may be fullyautomated thereby allowing rapid adjustment in the DUT measured at aleak test station within a production environment as well as optimizingthe test time and test determinations.

Determining DUT Leak:

Usually a leak tester is periodically checked using a master leak rateunit to ensure that the unit is within calibration. However, if the leakrate master unit is permanently connected to the leak test system thenan initial measurement of the leak rate master unit allows for averification step to be included within the test to ensure that thesettling and adiabatic heating times are over. The measurement of theleak rate will then indicate the leak of the system as a whole includingthe DUT. The leak of the tester is known so the leak of the DUT andhoses can be found.

Determine DUT Volume:

Systems are typically calibrated with a known leak standard as discussedabove on a periodic basis for calibration verification. However,according to embodiments of the invention this leak rate master unit canbe used to find the volume of the DUT thereby allowing the actual volumeof the part to be determined and employed in leak rate determinationsrather than using a design value derived from the engineering part.

Stabilization Step Removal:

As noted above the stabilization step of leak test processes can asignificant portion of the overall leak testing process cycle time. Ifthis effect is consistent then a model of this curve can be developedand can be subtracted from the original flow or pressure decay to yieldthe flow or pressure decay curves that are only due to the leak of thepart not including the adiabatic process. Accordingly, this would allowfor faster leak rate determination by allowing this to be performedearlier without requiring stabilization wherein the determination may bemade for example at a predetermined point in time or after a particularcondition has been met. An example of a process sequence according to anembodiment of the invention would be:

-   -   1. Run a representative part several times in a row capturing        the entire pressure and/or flow curves.    -   2. Align the curves using significant attributes, for example        threshold crossing at certain value for example, to remove any        timing variations.    -   3. Perform mathematical processing to develop a nominal curve        based on a point by point statistical analysis.    -   4. Save the nominal curve into memory.    -   5. During subsequent leak tests perform the alignment process on        the new test data.    -   6. Subtract the nominal curve from the new test data.    -   7. Determine the leak rate from adjusted test data.

8. Automatic Calibration in Tank to Tank Leak System:

In a Tank to tank flow base leak test system the source of the air is asealed tank that was filled to the same pressure as the DUT wherein aflow meter is disposed between the tank and the part. Accordingly if theDUT leaks some air comes out of the DUT and hence some out of the supplytank. The flow meter therefore only measures the flow out of the tankwhich is a portion of the actual part leak. In order to know the truepart leak from the flow meter measurement the DUT volume needs to beknown.

Accordingly employing the automatic parametric methods discussed abovein respect of embodiments of the invention then if these are applied toeach cycle to determine the DUT volume then the actual leak rate can beknown without having to know the DUT volume prior to the measurements.This therefore makes leak testing using the tank to tank method one ofreduced complexity, increased accuracy and removes the requirement for acalibration and according calibration time for each type of DUT.

9. Calculate Optimal Test Parameters from Sample Data:

Typically a leak test comprises a series of stages, such as for example,fill, stabilize, test and exhaust. Typically the values of these areestablished based upon subjective assessments of measurementcharacteristics or simulations. However, the inventors have found thatsome aspects of a test may be determined from statistical analysis of amultiple experiments such as for example stabilization time and testtime. According to an embodiment of the invention a leak test has beenestablished as having a known end time of 2.7 seconds wherein thefollowing procedure is executed:

-   -   1. Take various samples of data for a leak test;    -   2. Vary the test zone from the fill time end (0.5 seconds) to        the test end (2.5 seconds)    -   3. Calculate the mean leak and the standard deviation at each        point;    -   4. Plot the results    -   5. Chose the point that has the lowest standard deviation for        the time to start the test zone.

An exemplary plot of such a test protocol is depicted by FIG. 6 whereinthe plot presents the standard deviation of a test with fixed total testtime against the test start time indicating that a minimum exists atapproximately 1.1 seconds wherein the test time can be seen as part ofthe overall process in FIG. 7A.

10. Fit Curves to Leak Profile:

In many instances leak rate tests are performed at a single discretepoint in time or after a predetermined period of time where multiplemeasurements are performed with subsequent averaging. However, theinventors have established leak testers with high accuracy and fast dataacquisition such that data may be extracted from the test at high datarates thereby allowing the leak profile to be tracked in real time suchthat a mathematically defined curve may be fitted to the evolving leakprofile thereby increasing accuracy, reducing test times and allowingfinal leak data to be predicted.

For example FIG. 7B depicts the results from a leak test systemaccording to an embodiment of the invention such as depicted above inrespect of FIGS. 3 and 4 wherein a flow curve versus time is shownwherein a mathematical model, for example an exponential, is applied tothe data to determine the parameters by fitting the curve of theequation using the raw data. Accordingly, these parameters may then beused for example to increase accuracy or predict final leak rate beforestabilization.

11. Temperature Compensation Using Rate of Change of Flow/Pressure:

Within leak rate testing DUT temperature changes may impact directly thetesting such as for example if the DUT was heated in a previous process,i.e. high temperature washing, this results in a heating of the DUT suchthat subsequent cooling causes a flow into the DUT which is measured andattributed incorrectly to a leak within the DUT. Accordingly theinventors have established a protocol for determining the effects on theleak curve due to the temperature rate of change of the DUT from therate of change of the flow or pressure decay curve only. As the DUTcools the rate of change of the DUT temperature reduces such that with ahigh speed leak tester measurements can be made that allow this rate ofchange to be determined and therein from this the DUT temperature may beestablished. The duration over which the flow and/or pressure aresampled is set long enough for this to be determined.

Referring to FIG. 10 there is depicted a plot of corrected leak rateversus original leak value based upon a measured temperature changewhereas FIG. 11 depicts a plot of pressure vs. time curves for differentinitial DUT temperatures. The change in slope is highlighted for one DUTwith initial pressure slope 1110 and final pressure slope 1120.

Least squares slope may, for example, be used to determine the pressurerate of change. The rate of change of the pressure slope (doublederivative) is proportional to the temperature difference between theDUT and atmospheric or test stand temperature. The greater thedifference in temperature the greater the difference in rate of changeof the slope of the pressure. This value can then be used to determinethe part temperature and remove the temperature effect as shown in theresulting corrected leak rate values in FIG. 10. Referring to FIG. 11Bthe rate of change of pressure slope versus time for a DUT is depictedas raw pressure rate of change 1130 together with the initial and finalcalculated pressure rates of change 1140 and 1150 using least squaresfits to the initial and latter portions of raw pressure rate of change1130.

12. Signature Analysis of Entire Leak Curve:

Within the prior art a leak test is performed once a series of previoussteps have been completed such as fill and stabilization*n. However, theinventors have established a process to scientifically analyze theentire pressure-time curve of a pressure-decay or flow leak test withsignature analysis in order to extract all the information that can beobtained from each unique part of the waveform and by using as manycontinuously sampled points as possible. FIG. 12 shows examples of someof the parameters that are the result of full waveform analysis andwhich can be applied to not only leak testing as a qualification stepbut also subsequently retrieved in the event of any subsequent defectanalysis of the DUT or subsequent assemblies and/or systems exploitingthe DUT.

For example instead of only measuring the start and end pressure duringthe test time the invention uses a continuous stream of measurementstaken during the test time. The information from all these points isused to compute a much more reliable determination of the leak rate.This results in greater accuracy and faster leak tests. Accordinglyembodiments of the invention apply signature analysis to all parts ofthe pressure-time curve to obtain additional information about the testchamber and the test apparatus such as the status of the air regulators,valves, pressure transducer, connection to test chamber etc. For examplethe status of the hoses connected to the test chamber can be determinedby measuring the rate at which the DUT exhausts the air that was at thetest pressure through the exhaust valves.

For example, referring back to FIG. 7A a pressure decay leak test withinthe prior art is performed by filling a DUT with air and then measuringthe pressure at two different points in time, P₁ and P₂. The size of theleak in the DUT should then inferred by the difference between these twomeasured pressures and the time between them. Examples of such a priorart approach are Delatorre et al in U.S. Pat. No. 3,800,586 and Martinet al in U.S. Pat. No. 5,847,264. However, this type of analysis suffersfrom a common measurement problem in that the practical reading ofelectrical transducers has some degree of uncertainty, due to electricalnoise or other inaccuracies, so that simple two point measurements areprone to error. Also there is additional information in the entirewaveform or curve that is not being used and within each zone of theoverall leak test profile depicted in FIGS. 7A and 7B respectivelyinformation is present with respect to different paths for the airwithin the pneumatic leak test circuit and accordingly each zone andcombinations of zones can provide additional and different informationregarding the test and the status of the leak test system.

Accordingly embodiments of the invention relate to scientificallyanalyzing the entire pressure-time curve of a pressure decay or flowleak test with various combinations of signature analysis techniques forall the information that can be obtained from each unique part of thewaveform and by using as many continuously sampled points as possiblefor each analysis. Within the following discussion the approach ofsignature analysis for the entire leak test cycle from beginning to end(referred to as the entire leak curve) are presented with respect to anexemplary leak tester setup depicted in FIG. 13 which shows a typicalpressure decay leak test setup with air supply 1305, pressure regulator1310, dual valve arrangement comprising Valve 1 1315 and Valve 2 1320,pressure transducer 1325 and DUT 1330. Referring to FIG. 7A each zonerepresents different states of the test chamber and the controllingvalves wherein the status of each element within the LeakTester-Pneumatic Circuit 1300A are presented

TABLE 2 System State Table State Zone Valve 1 Valve 2 DUT System Check 0A A At Atmospheric Fill 1 B B Filling Stabilize 2 A A At TestPressure/Leaking Test 3 A A At Test Pressure/Leaking Exhaust 4 A BExhausting

12A. System Check:

Before the fill cycle even begins some system checks can be performed tovalidate the state of the sensors wherein the outputs of the transducersare collected for a small period of time and then analyzed using theLeak Tester-Controller Circuit 1300B for example comprising analogmeasurement circuit 1335, computer 1340, user interface 1345, andapplication software 1350. Accordingly FIG. 14 shows the system checkzone before the fill zone wherein within this exemplary embodiment thepeak to peak variation, PPZ0, and the average of the waveform, APZ0 aremeasured. The peak to peak variation PPZ0 may indicate excessively noisysensors, faulty excitation of sensors, bad wiring or valve problems forexample. The average APZ0 may indicate a damaged sensor, i.e. one thatwas over-ranged, or faulty valves that are not seated properly.Initially the thresholds for these two parameters may be establishedduring the execution of the multiple parametric excitation sequencessuch as described above in respect of Sections 6 and 7 for example.Alternatively, these thresholds may be based upon manufacturerinformation relating to the particular sensor(s) employed within theleak test system.

Other system checks that can be performed using a fully automated leaktester during the execution of a test cycle include for example analysisrelating to the maximum fill pressure, actual test zone start pressureand end exhaust pressure for example. The maximum fill pressure andactual test zone start pressure may for example indicate incorrectregulator setting(s) for the fast fill pressure or the fill pressure,incorrect supply line pressure, regulator functionality issues, supplyline hose integrity, gross defects, wrong part etc. The exhaust zone endpressure can indicate debris in hoses, kinked test port hoses or valvemalfunctions for example.

12B. Fill Zone—Pressure Chatter:

It would be evident that certain DUTs to be tested have internalsub-components and/or sub-assemblies that can move wherein this movementis not desirable. For example a DUT being tested may have O-rings withinit to form the seals wherein these O-rings may roll during the fillportion of the cycle as pressure differentials and pressure increasesoccur during this fill portion. When this happens there is a sharpreduction in the fill pressure due to the sudden movement of theO-rings. Other DUTs may have spool valves which are example of a movingsub-component that can move but in this instance move in a smooth way.If it is sticky the movement will be quick and uneven. Other examplescan be debris that becomes dislodged during the fill cycle or the faceseals of the leak apparatus are themselves deteriorating. However, suchinformation is not present in a prior art leak test but is presentwithin a full curve analysis. Accordingly to detect these sudden changesor discontinuities the fill portion of the pressure-decay versus timewaveform can be processed to generate an indication of the dynamicchange in pressure versus time. Accordingly this new waveform can beprocessed in order to determine maximum and peak-peak values for examplewhich can then be employed to indicate whether movement within the DUToccurred during the fill.

$\begin{matrix}{\frac{P_{n}}{t} = \frac{P_{n} - P_{n - 1}}{\Delta \; t}} & (2)\end{matrix}$

The pressure derivative determined from Equation (2) is a new waveformthat is generated by taking the difference between two adjacent pressuresamples and dividing this by the sample interval. If the minimum of thispressure derivative waveform is taken then this can represent the amountof movement of parts inside the test chamber. This is noted as CPZ1 forChatter Pressure in Zone 1 in the full curve profile presented in FIG.12. Referring to FIG. 15 it can be seen how the pressure derivativeclearly identifies such chatter within the DUT relative to the rawpressure versus time data. The processing applied to the resultingpressure derivative can be as simple as high/low limits with triggers todetermine if the DUT has acceptable movement or not. Even where limitscannot be initially established except through trial-and-error or areestablished based upon field data of DUTs then such data can be obtainedfor every piece part to establish a database from which such limits maybe derived or where limits are subsequently determined to identifyparticular DUTs for further monitoring or recall for example.

12C. Fill/Exhaust—Slope:

The fill and exhaust portions of the curve can also be analyzed todetect any anomalies in the part such as kinked hoses, reduced/expandedvolume, missing parts, blockages in DUT i.e. transmission housing, andgross leak for example. Some of the part of the curves can be modeled byexponential rise or fall so that a least squares curve fit to anexponential can yield good results. Equation (3) shows the basic form ofthe equation and FIG. 16 depicts an example of how it fits in the fillzone. Regulators and pneumatic devices exhibit nonlinear effects butthese can be molded using the combination basic elements. Thisrepresents a combined factor of the test volume, the resistance of theconnection to the test volume, and any leaks of system and part. Thetest volume is the combination of the leak tester test port volume,volume of connection hoses to the part and the volume of the part undertest. The resistance to fill is a combination of the air sourceresistance, regulator, valves, tubes and pluming associated with gettingthe air into the chamber. The resistance for the exhaust is acombination of the connection hoses, valves and the exhaust pathwaysleading the air out of the part to the atmosphere.

Pfall(t)=Pstart*e ^(−t/τ)  (3)

where P(t) is pressure at time t, P_(FINAL) is the final pressure, and τis the time constant of the fill process.

This slope or the time constant τ will be affected by any of the abovementioned defects or issues. For example, if the DUT is a brake line fora vehicle and the hose is kinked the pressure measured by the testerwill rise faster than it should for a good DUT. The pressure in the partwill rise slower, but the tester is measuring pressure before the blockso will see a faster rise in pressure than if there was no kink. Limitscan be set by testing initial populations of known good parts and badparts and noting the change in these slope features.

12D. Test Zone—Slope:

As noted above a leak test according to the prior art during the testportion of the pressure curve measures two points, one at the beginningand one at the end, and therein a leak rate calculated. However, withfull waveform signature analysis continuous sampling is performed alongthe test portion of the curve and these data points may be used todevelop a best fit exponential slope using a least squares technique onthe basis that the pressure profile will be an exponential decay fromthe starting pressure to ambient pressure (see FIG. 17). Thismathematical processing improves the accuracy of the measurements byfiltering measurement noise out and provides for reduced test asdiscussed below in Section 12E. The processing may employ the sameEquation (3) above.

12E. Test Zone—Early Rejection: Typically a leak tester according to theprior art must wait the prescribed test time and then performs thecalculation based upon the start and end pressures of the test zone todetermine the pass/fail status of the DUT. However, with full waveformsignature analysis continuous sampling is undertaken and theseadditional points are all used so accuracy is increased but also withthe increased accuracy early prediction becomes possible. According toan embodiment of the invention the leak test system may periodicallyextract the portion of the test pressure waveform that is available atthat point and execute mathematical processing on it thereby generatinga prediction on the final result as shown in FIG. 17. This final resultprediction as well as the current data portion of the test zone iscompared against predetermined limits. Accordingly, based upon earlysamples a prediction of a probable failure may be determined which uponreaching a predetermined level of confidence results in the DUT beingdeemed as a failure at that time, thus saving valuable production timeas the complete test cycle is not required. Alternatively parts may alsobe deemed to pass at earlier test times where the confidence levels ofthe predications are high. In this manner only parts with leak ratesthat are essentially borderline require completion of the full test zoneelement of the test profile.

Specific details are given in the above description to provide athorough understanding of the embodiments. However, it is understoodthat the embodiments may be practiced without these specific details.For example, circuits may be shown in block diagrams in order not toobscure the embodiments in unnecessary detail. In other instances,well-known circuits, processes, algorithms, structures, and techniquesmay be shown without unnecessary detail in order to avoid obscuring theembodiments.

Implementation of the techniques, blocks, steps and means describedabove may be done in various ways. For example, these techniques,blocks, steps and means may be implemented in some instances aselectronic circuits which may comprise hardware, software, or acombination thereof. For a hardware implementation, the processing unitsmay be implemented within one or more application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described above and/or a combination thereof.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flowchart, a flow diagram, a data flow diagram, astructure diagram, or a block diagram. Although a flowchart may describethe operations as a sequential process, many of the operations can beperformed in parallel or concurrently. In addition, the order of theoperations may be rearranged. A process is terminated when itsoperations are completed, but could have additional steps not includedin the figure. A process may correspond to a method, a function, aprocedure, a subroutine, a subprogram, etc. When a process correspondsto a function, its termination corresponds to a return of the functionto the calling function or the main function.

Furthermore, embodiments may be implemented by hardware, software,scripting languages, firmware, middleware, microcode, hardwaredescription languages and/or any combination thereof. When implementedin software, firmware, middleware, scripting language and/or microcode,the program code or code segments to perform the necessary tasks may bestored in a machine readable medium, such as a storage medium. A codesegment or machine-executable instruction may represent a procedure, afunction, a subprogram, a program, a routine, a subroutine, a module, asoftware package, a script, a class, or any combination of instructions,data structures and/or program statements. A code segment may be coupledto another code segment or a hardware circuit by passing and/orreceiving information, data, arguments, parameters and/or memorycontents. Information, arguments, parameters, data, etc. may be passed,forwarded, or transmitted via any suitable means including memorysharing, message passing, token passing, network transmission, etc.

For a firmware and/or software implementation, the methodologies may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. Any machine-readable mediumtangibly embodying instructions may be used in implementing themethodologies described herein. For example, software codes may bestored in a memory. Memory may be implemented within the processor orexternal to the processor and may vary in implementation where thememory is employed in storing software codes for subsequent execution tothat when the memory is employed in executing the software codes. Asused herein the term “memory” refers to any type of long term, shortterm, volatile, nonvolatile, or other storage medium and is not to belimited to any particular type of memory or number of memories, or typeof media upon which memory is stored.

Moreover, as disclosed herein, the term “storage medium” may representone or more devices for storing data, including read only memory (ROM),random access memory (RAM), magnetic RAM, core memory, magnetic diskstorage mediums, optical storage mediums, flash memory devices and/orother machine readable mediums for storing information. The term“machine-readable medium” includes, but is not limited to portable orfixed storage devices, optical storage devices, wireless channels and/orvarious other mediums capable of storing, containing or carryinginstruction(s) and/or data.

The methodologies described herein are, in one or more embodiments,performable by a machine which includes one or more processors thataccept code segments containing instructions. For any of the methodsdescribed herein, when the instructions are executed by the machine, themachine performs the method. Any machine capable of executing a set ofinstructions (sequential or otherwise) that specify actions to be takenby that machine are included. Thus, a typical machine may be exemplifiedby a typical processing system that includes one or more processors.Each processor may include one or more of a CPU, a graphics-processingunit, and a programmable DSP unit. The processing system further mayinclude a memory subsystem including main RAM and/or a static RAM,and/or ROM. A bus subsystem may be included for communicating betweenthe components. If the processing system requires a display, such adisplay may be included, e.g., a liquid crystal display (LCD). If manualdata entry is required, the processing system also includes an inputdevice such as one or more of an alphanumeric input unit such as akeyboard, a pointing control device such as a mouse, and so forth.

The memory includes machine-readable code segments (e.g. software orsoftware code) including instructions for performing, when executed bythe processing system, one of more of the methods described herein. Thesoftware may reside entirely in the memory, or may also reside,completely or at least partially, within the RAM and/or within theprocessor during execution thereof by the computer system. Thus, thememory and the processor also constitute a system comprisingmachine-readable code.

In alternative embodiments, the machine operates as a standalone deviceor may be connected, e.g., networked to other machines, in a networkeddeployment, the machine may operate in the capacity of a server or aclient machine in server-client network environment, or as a peermachine in a peer-to-peer or distributed network environment. Themachine may be, for example, a computer, a server, a cluster of servers,a cluster of computers, a web appliance, a distributed computingenvironment, a cloud computing environment, or any machine capable ofexecuting a set of instructions (sequential or otherwise) that specifyactions to be taken by that machine. The term “machine” may also betaken to include any collection of machines that individually or jointlyexecute a set (or multiple sets) of instructions to perform any one ormore of the methodologies discussed herein.

The foregoing disclosure of the exemplary embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many variations andmodifications of the embodiments described herein will be apparent toone of ordinary skill in the art in light of the above disclosure. Thescope of the invention is to be defined only by the claims appendedhereto, and by their equivalents.

Further, in describing representative embodiments of the presentinvention, the specification may have presented the method and/orprocess of the present invention as a particular sequence of steps.However, to the extent that the method or process does not rely on theparticular order of steps set forth herein, the method or process shouldnot be limited to the particular sequence of steps described. As one ofordinary skill in the art would appreciate, other sequences of steps maybe possible. Therefore, the particular order of the steps set forth inthe specification should not be construed as limitations on the claims.In addition, the claims directed to the method and/or process of thepresent invention should not be limited to the performance of theirsteps in the order written, and one skilled in the art can readilyappreciate that the sequences may be varied and still remain within thespirit and scope of the present invention.

What is claimed is:
 1. A method of leak testing a device under test(DUT) comprising the steps of: a) filling the DUT with a fluid to apredetermined pressure using a first regulator R2 and first valve V1disposed in series between a source of the fluid and the DUT; b)measuring with a high accuracy flow controller F1 a measured flowwaveform; and c) calculating with a microprocessor a leak rate for theDUT in dependence upon at least the measured flow waveform.
 2. Themethod of leak testing a DUT according to claim 1 wherein: step (b)further comprises; (i) measuring pressure with an absolute pressuresensor P1 at high resolution to generate an absolute pressure waveform,the absolute pressure sensor P1 disposed to measure pressure between thehigh accuracy flow controller F1 and DUT; ii) computing with themicroprocessor a derivative of the absolute pressure waveform; iii)computing with the microprocessor an effective flow due to the absolutepressure change waveform using the data obtained from the absolutepressure sensor P1 in dependence upon the known DUT volume and absolutepressure; and iv) subtracting this effective flow from the measured flowwaveform to generate corrected real flow waveform.
 3. The method of leaktesting a DUT according to claim 1 wherein: step (b) further comprises;i) waiting a predetermined period of time; ii) performing a highresolution measurement of the absolute test pressure using an absolutetest pressure sensor P1; iii) using the high resolution measurement ofthe absolute test pressure as a set point; iv) closing the fill valveV1; v) engaging a feedback loop between the absolute test pressuresensor P1 and a high accuracy flow controller; vi) measuring theresulting control flow signal from the high accuracy flow controller;and step (c) further comprises; vii) determining the leak rate for theDUT in dependence upon the resulting control flow signal.
 4. The methodof leak testing a DUT according to claim 3 wherein; step (b) furthercomprises; establishing a feedback loop between an absolute testpressure sensor P1 and a high accuracy flow controller, the feedbackloop comprising: providing a high resolution absolute pressure sensor P1disposed to measure absolute test pressure between the high accuracyflow controller F1 and DUT; providing a high resolution filtering andprocessing circuit for processing the high resolution measurement of theabsolute test pressure; and providing aproportional-integration-derivative controller to control the highaccuracy flow controller F1 based upon the output of the high resolutionfiltering and processing circuit.
 5. The method of leak testing a DUTaccording to claim 1 further comprising; d) determining whether to passthe DUT comprising the steps of: i) establishing a mathematical fit todata relating to the setting of the high accuracy flow controller over apredetermined period of time; ii) projecting the mathematical fitforward to a predetermined point in time; iii) determining whether theprojected result meets a predetermined criteria; and iv) associating aconfidence to the projected result in dependence upon at least one ofthe projected result, the predetermined period of time, and thepredetermined point in time; and v) passing the DUT when the projectedresult meets the predetermined criteria with a predetermined confidence.6. The method of leak testing a DUT according to claim 1 wherein; step(a) further comprises filling a reference volume to the same pressure asthe DUT; and step (b) further comprises; (i) sealing the referencevolume; (ii) employing a low full scale differential pressure sensor tomeasure the pressure difference between the reference volume and theDUT; (iii) controlling using a control loop the high accuracy flowcontroller F1, the control loop having the output of the differentialpressure sensor as the feedback input and a value of zero as the setpoint input such that the control loop has a large signal to noise ratioinput.
 7. The method of leak testing a DUT according to claim 1 wherein;step (a) comprises: (i) overfilling the DUT to an initial pressurehigher than a required final test pressure; (ii) maintaining the DUT atthe initial pressure until equivalent heat has been transferred to theDUT, the equivalent heat being that heat which would transferred to theDUT when filled to the final required test pressure when stablilized;(iii) reducing the pressure to the required final test pressure.
 8. Themethod of leak testing a DUT according to claim 1 wherein; step (a)comprises: (i) overfilling the DUT to an initial pressure higher than arequired final test pressure; (ii) monitor the current adiabatic rate ofheat transfer; (iii) adjusting the DUT pressure from the initial overfill pressure to required final test pressure using adiabatic rate ofheat transfer as the input; (iv) controlling the pressure such that therequired final test pressure is reached with an adiabatic rate of heattransfer below a predetermined threshold.
 9. The method according toclaim 1 wherein; at least one of: at least one of a pressure regulatorR1 attached to an inlet for receiving the fluid and a test regulator R2disposed before the first valve V1 is an absolute pressure regulatorthereby reducing error due to absolute atmospheric pressure changes; andthe calculated leak rate for the DUT is corrected by a correction factorestablished in dependence upon at least the DUT temperature, the DUTtemperature being established in dependence upon a calculated rate ofchange of the pressure slope measured with an absolute pressure sensorP1 disposed to measure pressure between the high accuracy flowcontroller F1 and the DUT.
 10. A method of leak testing a device undertest (DUT) comprising the steps of: a) fast filling the DUT with a fastfill regulator R2 via a fast fill valve V1 at a pressure higher than therequired final test pressure; b) reducing the pressure of the DUT usingfast fill regulator R2 to required final test pressure; c) measuring DUTpressure using an absolute pressure sensor P2; d) setting a controlset-point of an absolute test pressure regulator R1 in dependence uponthe measured DUT pressure; e) filling the DUT using absolute testpressure regulator R1 via a fill valve V5 after closing fast fill valveV1; and f) switching to flow testing with high accuracy flow controllerF1 via flow text valve V3 after closing fill valve V5; g) measuring withthe high accuracy flow controller F1 a measured flow waveform; and h)calculating with a microprocessor a leak rate for the DUT in dependenceupon at least the measured flow waveform.
 11. The method of leak testinga DUT according to claim 10 wherein; at least one of: steps (a) though(c) are performed twice using different pairs of fast fill regulator R2and fast fill valve V1; and steps (a) through (e) are performed twiceusing different sets of fast fill regulator R2, fast fill valve V1,absolute test pressure regulator R1 and fill valve V5.
 12. A system fortesting a device under test (DUT) comprising: an absolute pressuresensor in communication with a fluidic system to be coupled to the DUT;a high accuracy flow controller and high resolution flow sensorcomprising a predetermined portion of the fluidic system and disposedbetween a fluid source and the DUT; a high resolution low noise dataacquisition circuit coupled to the absolute pressure sensor forreceiving the output of the absolute pressure sensor and converting itto an electrical signal; and a controller receiving the output of thehigh resolution low noise data acquisition circuit and controlling theflow of the high accuracy flow controller with integral flow sensor independence upon at least the processed output of the absolute pressuresensor.
 13. The system according to claim 12 wherein; at least one of:the controller is a proportional-integration-derivative controller; thehigh resolution low noise data acquisition circuit filters and processesthe absolute pressure sensor output using software based processing; andthe high accuracy flow controller and high resolution flow sensor areintegrated into the same unit.
 14. The system according to claim 12further comprising; a regulator R1 disposed between the fluid source andthe high accuracy flow controller with integral flow sensor; and a testregulator R2 and fill valve V1 in series and disposed between theregulator R1 and DUT in parallel to the high accuracy flow controllerwith integral flow sensor.
 15. The system according to claim 12 furthercomprising; a computer coupled to the high resolution low noise dataacquisition circuit and controller executing a software application fordetermining i) a mathematical fit to data relating to at least one ofthe absolute test pressure sensor output and the setting of the highaccuracy flow controller with integral flow sensor over a predeterminedperiod of time; ii) projecting the mathematical fit forward to apredetermined point in time; iii) determining whether the projectedresult a predetermined criteria based; and iv) associating a confidenceto the projected result in dependence upon at least one of the projectedresult, the predetermined period of time, and the predetermined point intime; v) determining whether to pass the DUT.